Emulsion polymer systems of butadiene-styrene, vinyl acetate-ethylene, acrylics and other vinyl monomers have been formulated into systems suited for making nonwoven binder systems. Typically, these systems will contain a cross-linkable monomer interpolymerized therein so that increased wet and dry strength and solvent resistance can be achieved by the nonwoven binder. Vinyl acetate-ethylene copolymer systems containing N-methylolacrylamide, ethers of the methylolamide or carboxyl functionality have been utilized in this field.
Heretofore, the industry has generally used a three-component redox initiator system for effecting polymerization of the monomers mentioned. These systems comprise a catalyst (oxidizing agent) such as t-butyl hydroperoxide, persulfates or hydrogen peroxide, an activator (reducing agent) such as an alkali metal bisulfite or an alkali metal formaldehyde sulfoxylate, and a transition metal ion moderator. One of the problems with such a system is that formaldehyde is present in the common reducing agents and is a cancer-suspect chemical. Thus, the industry is seeking ways to avoid its use in forming polymer systems used in the manufacture of nonwoven goods which come in contact with humans such as paper products, paper towels and baby diapers. Various initiator systems including formaldehyde free systems are described in the following patents.
U.S. Pat. No. 4,094,849 discloses a process for producing ethylene-vinyl acetate copolymer containing no formalin by emulsion polymerizing ethylene and vinyl acetate with a redox system consisting of an oxidizing agent, such as hydrogen peroxide or t-butyl hydroperoxide, and a reducing agent consisting of the reaction product of a glyoxal compound and an alkali metal, ammonium or zinc salt of a reductive sulphur oxide, e.g. a thiosulfuric acid, dithinous acid or a pyrosulfuric acid or bisulfite. Glyoxal bisulfite is specifically used as a reducing agent.
U.S. Pat. No. 2,716,107 discloses a process for producing synthetic rubber emulsions utilizing a redox system consisting of an iron salt, an alkali metal salt of ethylenediaminetetracetic acid, a peroxide, and a reducing agent consisting of an alkali metal ketone sulfoxylate. A mixture of acetone sulfoxylate-acetone bisulfite is used as the reducing agent and is formed by reacting acetone with sodium dithionite. The molar ratio of sodium acetone sulfoxylate to sodium acetone bisulfite produced by this reaction is 1:1. It was reported that the bisulfite did not interfere with the action of the sulfoxylate in accelerating the polymerization reaction rate and of itself did not have any appreciable polymerization accelerating action.
U.S. Pat. No. 3,637,626 discloses a process for polymerizing vinyl chloride utilizing an initiator consisting of an organic hydroperoxide, an alcoholate, an organic sulfite and sulfinic acid. Examples of organic sulfites include dimethyl and diethyl sulfite.
U.S. Pat. No. 2,560,694 discloses a process for polymerizing acrylonitrile using sodium bisulfite as the water-soluble reducing agent. Inorganic bisulfites, aliphatic aldehyde bisulfite adducts, sulfinites, inorganic thiosulfates and sulfoxylates and bromate ion where there is a molar excess of bromate ion, are effective catalysts for low temperature polymerization.
U.S. Pat. No. 3,700,456 discloses a process for producing photographic silver halide emulsions by polymerizing vinyl monomer in the presence of a redox system containing bromate or chlorate ion as the oxidant and a bisulfite reducing agent, e.g. potassium and sodium bisulfite.
U.S. Pat. No. 2,383,055 discloses a process for polymerizing conjugated dienes using a redox system where the reducing agent consists of sulfites, bisulfites, sulfoxylates, etc. Sulfur dioxide, sodium sulfite and sodium bisulfite are used as the reducing agents.
German Ausl. No. 2,456,576 describes the use of a bisulfite with a persulfate or t-butyl hydroperoxide in a continuous polymerization process for vinyl acetate-ethylene emulsions, but requires that the free monomer be below 15% and the bisulfite be used in 4-10 fold excess.